Fibs (fibrilations) are spontaneous electrical activity of individual muscle fibers (muscle fibers are what make-up our muscles) when they become denervated. Positive sharp waves (PSW's) are thought to be the same thing. They are both a sign of active denervation.
They can both disappear if the muscle fibers have been reinnervated and there is no longer active denervation. If reinnervation has occured and active denervation has ceased, all that would be seen on the EMG is an increase in the size of motor unit action potentials (MUAP's). An increase in the size of MUAP's is a sign of reinnervation. Only if denervation is active while reinnervation is also occuring, will you see fibs / PSW's and an increase in the size of MUAP's at the same time on the EMG.
Just for the record: you can have the above scenario with ALS (i.e. if El Eschorial criteria are met) but you can also have it with many other conditions, too.